The present invention relates to a method of duplex copying by use of an electrophotographic duplex copying machine capable of forming images on both sides of each transfer sheet.
In the conventional method of duplex copying, as illustrated in FIG. 1, a sheet original or a bound material, such as a book, is placed on a contact glass 1 and a photoconductive drum 3 is exposed to a light image of the original by the scanning of an exposure optical system 2 so that a latent electrostatic image corresponding to the light image is formed on the photoconductive drum 3. The thus formed latent electrostatic image is then developed by a development apparatus 4 and the developed image is transferred to one side of a transfer sheet fed from a transfer sheet feed apparatus 5. After the transferred image is fixed to the transfer sheet by an image fixing apparatus 6, the sheet is stacked on a secondary transfer sheet feed apparatus 7, with the image-bearing side up.
Another original is then placed on the contact glass 1, and taking the same procedure as in the above-mentioned case, a latent electrostatic image is formed on the photoconductive drum 3 by the scanning of the exposure optical system 2. The transfer sheet which has had an image fixed on one side thereof, and which has been stacked on the secondary transfer sheet feed apparatus 7, is then fed from the apparatus 7 in such a manner that the other side thereof is brought into contact with the photoconductive drum 3. Thus, a toner image corresponding to an image of the second original is transferred to the back side of the above-mentioned transfer sheet and is discharged to a transfer sheet output tray 8 with its second side up after the transferred image has been fixed by the image fixing apparatus 6.
Thus, in the conventional method of duplex copying, two copying processes are required for one duplex copying.
Even in the case where a duplex copying machine having an auto document feeder for use with sheet originals is utilized, the same copying process has to be repeated two times, as in the above case, for forming images on both sides of each transfer sheet. One copying process means, in this specification, a copying process consisting of formation of a latent electrostatic image on a photoconductive drum by one scanning and exposure, development of the latent electrostatic image, transfer of the developed image to a transfer sheet after fixing of the developed image, and discharging of the image-bearing transfer sheet.
Recently speed-up of duplex copying is greatly demanded and, accordingly, various attempts have been made to develop a high speed duplex copying machine. However, a satisfactory duplex copying machine capable of making duplex copies speedily has not been developed.
Moreover, in duplex copying, transfer sheets having had images farmed on one side thereof have to be transported again to an image transfer station for duplex copying. In this case, it is important that such transfer sheets are stacked neatly and in good order. Otherwise, copying of other sides of the transfer sheets cannot be performed in a proper position of the respective transfer sheets. However, in the conventional duplex copying machine, neat stacking or lineup of such transfer sheets is not made.